1. Field of the Invention
This invention relates to integrated circuit voltage references, and more particularly to zener diode voltage references capable of being implemented with CMOS (complementary metal oxide semiconductor) processing techniques.
2. Description of the Prior Art
Voltage references are required to provide a substantially constant output voltage irrespective of changes in input voltage, output current or temperature. Such references are used in many design applications, such as stable current references, multipliers, control circuits, portable meters, two-terminal references and process controllers.
It would be desirable to have better voltage references for devices formed with a CMOS process. The benefits of CMOS for both analog-to-digital (A/D) and digital-to-analog converter (DAC) products have resulted in their use in many new designs. The processing for these circuits must solve linear high-accuracy problems as well as provide high densities for the required digital-logic circuits. Unfortunately, the development of voltage references which use CMOS processing and are relatively insensitive to temperature changes has not kept up with other CMOS developments.
Modern voltage references are generally based on either zener diodes or bandgap generated voltages. When implemented in CMOS, bandgap voltage references have been found to require relatively complicated designs, generally including at least two operational amplifiers. While zener reference circuits are simpler in design, they generally require the use of other diodes that are not available with CMOS processing. Parasitic bipolar transistors can be achieved with CMOS, but they have not been found to be capable of effective use as diodes.